Apparatus to permit both contouring and numerical positioning operations with a common control system



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o A M um Y 4 UT... E wm. mmmwl Emm from @zu f WV 1V TV 3 3 sa p n o -um wzl United States Patent APPARATUS TO PERMIT BOTH CONTOURING AND NUMERICAL POSITIONING OPERATIONS WITH A COMMON CONTROL SYSTEM John T. Evans, Waynesboro, Va., assignor to General Electric Company, a corporation of New York Filed July 8, 1963, Ser. No. 293,384 16 Claims. (Cl. 23S-151.11)

This invention relates to electronic .automatic control systems, and more particularly, to numerical control systems for controlling the positioning of a cutting element of a machine tool relative to a workpiece.

Heretofore, machine tool control equipment has been considered to fall into the separate categories of Numerical Contouring Control systems and Numerical Positioning Control systems. Numerical Positioning Control primarily differs from Numerical Contouring Control because positioning solely requires a command containing information as to the ultimate location of a workpiece relative to a cutting element, whereas Contouring requires commands containing information as to the irate of speed and the instantaneous direction of motion of a workpiece relative to `a cutting tool. The present invention relates to equipment designed to permit both Contouring oper-ations and numerical positioning operations with a common control system.

Accordingly, an object of the present invention is to provide unique means operative to position apparatus in response to absolute positioning data and to afford controlled feed rate between a present position and a new position in order to accomplish Contouring operations.

Although a large number of systems have been developed for numerical control of positioning apparatus, the present invention belongs to that class of numerical control systems wherein 4the commanded position of the apparatus and the actual position of the apparatus are accurately represented by the phase of a command and position signal, respectively. In this class of system, the apparatus is positioned in accordance with the difference in phase.

In general, the control data which determines the final position of the equipment .and the speed with which the equipment must move to attain that position, is presented to the control system in numerical form programmed on punched tape or punched cards; although in certain applications, magnetic tape containing the digital information may be used. The numerical input data is routed to appropriate subsections of the control system, wherein the control function is set into operation. In order that the numerical information be utilized by the electronic control equipment, the input data must be presented in an electrical form compatible with the over-all system and which will enable the control system to accurately control the speed and ultimate position of the machine tool relative to the workpiece. One form of representation of the position of equipment is a phase-coded signal having a discrete phase displacement with respect to a reference signal. With this form of representation, speed is represented by the rate at which the phase of such a signal is varied.

In typical Numerical Contouring Control systems, the velocity of motion lis determined by the rate of phase change of a pulse train and the distance to be traveled is determined by the number of pulses in the train. In typical Numerical Positioning Control systems, the commanded position is Vrepresented by the phase difference between a command signal and a reference signal, each of which are generated in response to pulse trains. In contradistinction to these systems, in the present invention, position is represented by the phase of a first comrace mand and the velocity of motion in attaining the position is represented by the rate of phase change of a second command.

Another object of the present invention is to provide numerically controlled equipment for generating a first and a second command for respectively establishing a preselected apparatus position and the rate at which the apparatus is to proceed while attaining the preselected position.

It is found advantageous to permit an operator to selectively determine whether the apparatus should position (1) in -accordance with a fixed velocity program, (2) in accordance with a controlled velocity program (e.g., by taped commands), or (3) in accordance with a modified version of a controlled velocity program. Thus, if the positioning capabilities only of the controlled machine are being used on a particular job, operation (l) may offer the most efficient utilization of the machines capabilities. On the other hand, if the job requires certain machining to take place while the machine is being positioned from a first to a second location, operation (2) becomes almost a necessity. Further, although a controlled velocity program may be available, there are times when a trained operator can recognize the need for a variation in the commanded velocity in order to insure proper functioning of all equipment involved. It is in the latter situation that operation (3) provides an important feature.

Another object of the invention is to provide an improved numerical control system wherein equipment may be accurately positioned either in accordance with a fixed velocity program, a controlled vel-ocity program, or a modified controlled velocity program.

In the system described hereinafter, a sensing mechanism observes the position of the machine tool as the machine tool responds to command signals, and generates a signal Whose phase relative to the reference is representative of the actual position of the machine tool. The desired position the machine tool is to yassume is represented by the phase difference between a first command signal and this feedback signal. The velocity with which the machine is t-o traverse the distance from its initial position to the desired position is represented by the rate of change of phase of a second command signal which is initially synchronized with this feedback signal.

In effect, two control circuits are used for each controlled motion. In a first of these control circuits, the ultimate position is determined by means for comparing the feedback signals with a position command signal. This comparison generates an er-ror signal which may be used to command automatic traverse to the commanded position in accordance with a predetermined rate program. In the second control circuit a controlled velocity program is implemented by means for generating a feedrate-position command signal which is initially in phase with the feedback signal and thereafter varying this feedrate-position command signal at a rate commensurate with any desired velocity.

Another object of the present invention is to provide equipment with cooperating control circuits for one or more axes of motion which determine both the final position of a machine element and the rate of traverse thereto.

When the position an apparatus is to assume and the velocity of motion it is to use to attain that position are represented by independent signals, it is necessary to recognize when the apparatus has moved into proximity with the commanded position and remove the velocity control before the apparatus passes the position. Simple positioning is accomplished by automatically moving the apparatus at a fixed traverse rate until it comes within a predetermined zone surrounding the commanded position and then decelerating at a rate proportional to the error within the zone. Velocity controlled positioning is accomplished herein by moving the apparatus at the controlled velocity until within the described predetermined zone and even thereafter, until the controlled velocity is equal to the velocity dictated by the position error.

Still another object of the present invention is to provide improved means for selectively controlling the positioning rate of a machine element until the velocity is equal to a velocity commanded by the actual position error and thereafter automatically imposing a positioning rate proportional to the difference between the actual position and the commanded position.

In accordance with the present invention, velocity controlled operation is implemented by generating a feedrateposition command signal in synchronism with the feedback signal representing the current position of the apparatus and varying the phase of this feedrate-position command signal at a rate commensurate with the desired apparatus feedrate. During such operation, the apparatus responds to the comparison between the feedrate-position command signal and the feedback position signal and approaches the desired position at the described rate. Once the apparatus attains a position within a predetermined zone surrounding the desired position and when the feedrate-position signal and command position signal are synchronized, the control over the rate of apparatus positioning is relinquished to the position error as determined by a comparison of the command position signal and the feedback position signal. This type of control insures more accurate control over apparatus movement than has heretofore been attainable with prior velocity control systems.

The novel features of the invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further advantages and features thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings of an illustrative embodiment of the invention wherein:

FIGURE 1 is a general block schematic showing the basic components present in a numerical control system embodying the features of the invention;

FIGURE 2 is a somewhat more detailed block schematr-ic drawing illustrating the general components employed in providing the novel features of the invention as embodied in the control section for a single axis of machine motion;

FIGURES 3A through 3H illustrate conventional logic symbols used in the. following circuit schematics and truth tables for describing the operations performed by the circuits represented;

FIGURE 4 is a logic schematic of circuitry for generating reference signals and position command signals;

FIGURE 5 is a logic schematic of a Positioning Velocity Control' circuit for developing a pulse repetition rate commensurate with a numerical command;

FIGURE 5A co-mprises timing diagrams descriptive of the Positioning Velocity Control circuit operation;

FIGURE 6 is a logic schematic of a Manual Feedrate Override circuit for manually modifying the output pulse repetition `rate from` the Positioning Velocity Control circuit in steps of FIGURE 6A comprises timing diagrams descriptivefof the operation of the Manual Feedrate Override circuit;

i FIGURE 7 is a general logic schematic of a Feed Command Phase Counter for generating a` phase-coded command signal that varies in phase at a rate commensurate with the commanded positioning velocity;

FIGURE 7A is a more detailed logic schematic of aA Feed Phase Counter;

FIGURE 7B comprises a truth table illustrating the permutations of state of a typical count-up binary-coded decimaldecade;

FIGURE 7C is a timing diagram illustrating the operation of a Feed Command Phase Counter;

FIGURE 8 is a logic schematic of circuitry for detectving the relationship between the command and position signals and for detecting the presence of the apparatus within a number of zones surrounding the commanded position;

FIGURES 8A, 8B, and 8C comprise timing diagrams for use in conjunction with an explanation of the system control circuitry;

FIGURE 9 is a logic schematic of a Phase Discriminator that generates feed signals for controlling the velocity with which the feed mechanism drives the apparatus;

FIGURES 9A, 9B, 9C, and 9D comprise timing diagrams descriptive of the operation of the Phase Discriminator under selected conditions; and

FIGURE l0 is a logic schematic of a Feed Allowed Circuit for detecting synchronism between the position command signal and the velocity command signal.

In order to provide a succinct description of an embodiment of the invention, a number of individual subcircuits are hereinafter described in detail in conjunction with specific figures. The entire control system Will first be generally described and thereafter the operating components of the system which are believed to require explanationl in order to permit one skilled in the art to practice the invention are treated4 in greater detail. The specification is arranged in accordance with the following table of contents:

TABLE OF CONTENTS General Description Detailed Description- Circuit Symbology Position Command Signal Generation (FIG. 4) Positioning Velocity Control Circuit (FIGS. 5, 5A) Manual Feedrate Override (FIGS. 6, 6A) Feed Command Phase Counter (FIGS. 7, 7A, 7B, 7C) Phase Comparators and Command Larger Detector (FIGS.

8, 8A, 8B, 8(9).- 26 Discriminator-Automatie Positioning Operation (FIG. 9) Discriminator-Veloeity Controlled Operation (FIGS. 9, 9A, 9B, 9G, 9D) Feed Allowed Circuit (FIG. l0); Summary and Claims GENERAL DESCRIPTION FIGURE 1 contains a general block diagram of `a control system of the nature contemplated. For convenience, the machine tool tobe controlled is illustrated by a block 1-10 at the right central portion of the figure. It should be understood that the teachings of the invention are applicable to any machine control wherein the positioning of an operating machine element with respect to a workpiece is of importance.

The function of the entire system, as illustrated in FIGURE 1, is to control machine tool1-10` automatically in response to numerical data read. from the numerical data input equipment 1-14 appearing on the left ofthe figure. Machine tool 1-10` comprises the machine element 1`13 to be controlled by the control section,l the X-axis feed mechanism 1-11, andthe Y-axis feed mechanism 1-12. The feed mechanisms 1-11 and 1-12 comprise appropriate drive shafts and gearing which actuate machine element 1-13 for motion along the X and Y axes. Obviously, the system may be utilized for controlling machine elements in additional coordinates, butto simplify the explanation of the principles of the invention, a description of the third coordinate has been omitted. Machine element 1-13 may be the cuttingtool itself or it may be the table holding the workpiece which is to :be operated upon. Alternatively, feed mechanisms 1-11 and 1-12 may control both the cutting tool and the motion of the Work.

A block of information on the punched tape, in accordance with the system to be described, contains al1 of the information necessary for a positioning and feeding operation. The data is presented in words, each of which has a letter :address as the initial character. The characters in each word are made'up of a plurality of simultaneously read elements encoded in the well known binary form. An example of a word calling for a position on the X-axis might be Xl23456, where each of the characters is represented in binary form. The letter address X indicates that the following numerical characters represent a position on the X-axis. Consequently, when this letter address is detected, the following numerical characters are routed to the X-axis control section of the control system for generation of command signals.

In accordance with the velocity control features of the present invention, data words are also presented which indicate the velocity with which the machine tool element 1-13 is to move with respect to the workpiece. The word designating the particular velocity desired is also made up of a plurality of characters presented in binary for-m.v Thus, a complete command for one axis of motion includes a word commanding a desired position and a word commanding the rate at which machine tool element -13 is to move in order to attain that position.

Generally speaking, the position data is used to control a position command phase generator such as X-axis position command phase generator 1-22 and the velocity data is used to control ,a feed command phase generator such as X-axis feed command phase generator 1-23. The position command generator 1-22 functions to provide a command signal Whose phase with respect to a reference signal from a pulse rate divider 1-28 represents the desired position machine element 1-13 is to assume. The feed command generator 1-23 functions to provide a command signal which is initially in phase with a sig-nal representing the instantaneous machine position .and continuously changes at a rate commensurate with the desired velocity of traverse.

Before proceeding with -a consideration of the processing of input data to develop command signals, it is worthwhile to consider the servo -loops which are involved in the control of each axis of motion rof the machine control element 1-13. The X-axis and Y-axis servo loops are structurally and electrically independent of each other in their action of driving the feed mechanism. Since the equipment throughout the system for the X-coordinate is precisely the same as for the Y-coordinate, solely the X-coordinate control section will ibe described. As shown in FIGURE l, corresponding elements of the Y-axis control section have been given the same numerical designation las those of the X-axis control section. They are distinguished by a prime signal The X-coordinate servo loop comprises an X-axis position servo 1-20, including a DC amplifier driving a servo motor which by its output shaft 1-19 controls a feed motor control to actuate the X-axis feed mechanism 1-11. Simultaneously, position servo shaft 1-19 drives the X- axis multiple range position feedback resolvers 1-18. The output leads 1-27 of the multiple range position feedback resolvers provide an electrical representation of the position of the machine in the X-coordinate since both the feed mechanism 1-11 and the multirange position feedback resolvers 1-18 are driven in common by the position servo 1-20. Leads 1-25 are coupled into the X-axis phase discriminator 1-21. The function of the X-axis phase discriminator is to compare the position signal applied lover lead 1-25 with command signals on leads 1-26 and 1-27 from the X-axis position command phase generators 1-22 and the X-axis feed command phase generator 1-23, respectively. By comparing the phases of the command signals and the feedback position signal, an error signal is developed which is fed into the servo 1-20 for driving the X-axis feed mechanism.

A fundamental element in a phase control system such as contemplated herein is the reference pulse generator -15. This generator produces a train of pulses having a predetermined repetition rate. It provides the carrier by which the command signals are transported throughout the control section; it is also used to develop synchronized pulse trains .of selected repetition Irates for use throughout the control system. Thus, the output of reference pulse generator 1-15 is applied via lead 1-16 and pulse rate divider 1-28 to the multiple range position feedback resolvers 1-18 and is also applied over lead 1-7 to the command phase generators 1-22 and 1-23.

The `output of pulse rate divider 1-28 represents a standard signal and the phase deviation between this standard signal and the output of the X-axis position command phase counter 1-22 represents the absolute position the machine element 1-13 is to assume with respect to a predetermined reference point. The standard signal at the output of pulse divider 1-28 is applied to the X-axis position feedback resolvers 1-18 (via known adapting circuitry which is omitted for brevity) which impart to it a phase shift that is commensurate with the instantaneous position of machine element 1-13. The output of feed command phase counter 1-23 is initially synchronized with this position signal and thereafter varies in phase at a rate determined by the input `data and commensurate with the velocity at which the machine element is to move.

Two distinct types of operation are possible with the control system shown in FIGURE 1. In a first type of operation, the numerical data input equipment 1-14 supplies position data to the X-axis position command phase generators 1-22 which thereupon generate phase-modulated outputs on lead 1-26 representative of the commanded position. The X-axis phase discriminator 1-21 functions in response to these command signals and the position signals and the position signals on lead 1-25 to drive the X-axis position servo 1-20 in accordance with a predetermined rate program until correspondence between the command and position signals occurs. At this time, machine element 1-13 is positioned in accordance with the input data.

As already noted, one of the unique contributions of the present invention is the ability to in effect perform contouring operations with equipment compatible with the above-described positioning operations. This is accomplished by with data indicative of the veloclty with which machine tool element 1-13 1s to be moved. Under the control of velocity control circuit 1-24, X-axis feed command phase generator 1-23 develops a phase-modulated output on lead 1-27, the phase of which varies with respect to the equipment is to move. In this mode of operation, the X-axis phase discriminator 1-21 drives the X-axis position servo 1-20 at a velocity determined by this rate of change until a position in proximity to the desired end position is attained. At that time, upon correspondence between the output of feed command phase generator 1-23 and the controlling position command phase generator 1-22, phase discriminator 1-21 directs nal positioning in accordance with the phase difference between the outputs of the position command phase generator and the corresponding position feedback resolver 1-18.

A more complete understanding of the unique features of the invention may be gleaned from a consideration of the more detailed block schematic in FIGURE 2. This block schematic illustrates the control equipment which would be used for controlling a single axis of machine motion. The command phase generators 1-22 and l-23, the phase discriminator 1-21, and the position feedback resolvers 1-18 have been shown in terms of their component parts. The description to follow will be concerned only 'with a single axis control system inasmuch as the other coordinates in motion are controlled by substantially similar circuitry.

With respect to FIGURE 2, and in understanding the following description, the numerical data input equipment 2-14 should be conside-red to comprise the various components of such equipment required to provide usable control data. For example, the input equipment might comprise a tape reader, a number recognition means for recognizing numerical characters, a letter recognition means for recognizing letter characters, and a sequencing or control means for developing the necessary sequencing signals required to control information read-in and circuit cycling in response to the input data. The particular manner in whichsuch functions are implemented is not material to the invention herein and any of the suitable ways known in the art may be employed. In the following description it is assumed that control information for the illustrated axis control system has been presented and stored in the appropriate control sections.

In general, as new command data is presented by control tape equipment 2-14, the individual numerical characters are preset into a read-in counter 2-19. After being set, the read-in counter 2-19 operates to produce a series of pulses equal to the number preset therein. These pulses are steered by distributor 2-20 to the counting inputs of the appropriate decades of the command phase counters 2-11, 2-12, and 2-13. Three separate command phase counters are used to generate three components of the six digit position command assumed for use in the illustrated system. (Such a command signal will permit positioning over a range of 100 inches with an accuracy of 0.0001 of an inch.) Each command phase counter is a binary-coded count-up circuit operative to assume 1000 discrete permutations of output condition. Furthermore, each command phase counter comprises three separate decades which are operative in binary-coded-decimal form to count from l to l0. During this initial presetting phase of operation, the three least significant digits of the command are preset in fine command phase counter 2-13. The three most significant digits are preset in coarse command phase counter 2-11, and medium command phase counter 2-21 is preset with data derived from the intermediate four digits.

When all input dat-a is preset into the command phase counters 2-11, 2-12, and 2-13, clock pulses are applied thereto from reference pulse generator 2-15. Assuming the pulses to have a repetition rate of 250l kilocycles per second, t-he counters will divide this input by 1000 and furnish 250 cycle per second square wave outputs. The phase of these output signals is displaced in proportion to the command number stored therein from a similar square wave output produced by a straight divide-by-lOOO counter. 'Ihe reference pulse rate divider 2-17 is in fact such a divide-by-l000 counter and consequently the phase differences between the outputs of phase counters 2-11, 2-12, and 2-13 and the output of pluse rate divider 2-17 represent the commanded position in a coarse, medium, and fine range, respectively.

In order to direct the positioning equipment, the control signals must be compared with corresponding signals representing the actual machine position. To develop such corresponding signals, the reference voltage ouptput from pulse rate divider 2-17 is applied via resolver supply 2-18 to multi-range position resolvers 2-32, 2-33, and 2-34. Resolver supply 2-18 converts the input thereto into suitable sine and cosine voltages for application to the orthogonally disposed windings of the resolvers. By way of example, the resolvers 2-34, 2-33, and 2-32 may be geared, as represented by dashed lines 2-35, to turn one revolution for eac-h of the respective distances of machine travel 0.1 of an inch, 2.0 inches, and 100 inches, thereby generating a tine, medium, and coarse range of feedback data. In response to the excitation from resolver supply 2-18, each resolver produces a single phase output signal having a phase relative to its input that varies as a function of its shaft position. These outputs are easily converted to square waves by waveshapers (not shown) for comparison with the command signals in the corresponding ranges.

It has thus been established that coarse resolver 2f-32 produces a 250 cycle per second signal having a phase representative of the apparatus position within a inch range; medium resolver 2-33 produces :a 250 cycle per second signal having a phase representative of the apparatus position within a 2 inch range; and fine resolver 2-34 produces a 250 cycle per second signal having a phase representative of the position of the apparatus within a 0.1 of an inch range. These signals are individually applied to coarse comparator 2-26, intermediate comparators 2-27, and phase discriminator 2-29, respectively. In addition, each of the signals from both the resolvers and the command phase counters is applied to a command larger detector 2-28 wherein the determination is made as to whether or not the equipment must be moved in a forward or reverse direction in order to attain the position commanded by the input data.

When numerical positioning solely is contemplated, and nocontouring is desired, the positioning feed mechanism is driven at a constant rate of speed over the major portion of the distance to be traversed. For this reason, the generation of analog voltages proportional to the error between two widely divergent positions is generally unnecessary. In the illustrated embodiment as described more fully in connection with FIGURE 8, end zones are created within which special consideration 4is given to the phase difference between the particular command and position signals, and outside of which, only a basic determination is made of which direction of movement is required to attain a commanded position. For relatively large differences between the com-mand and position signals, the coarse comparator 2-25 controls the feed mechanism to drive in either required direction at a predetermined constant rate of speed until the apparatus cornes within a preselected coarse end zone. Within this zone, comparison of the medium command and position signals is made by intermediate comparators 2-27 to accurately determine the direction of traverse. Thereafter, when within a more sharply defined end zone, the tine command and feedback signals are compared in phase discriminator 2-29 to develop an analog signal having a magnitude proportional to the amount of error. In the positioning mode of operation; the Imachine feed mechanism and the control system are'designed to cooperate completely without developing more information than is necesssary, and with the necessary information being developed as economically and efficiently as possible.

In order to perform contouring operations, -it is necessary to control the velocity of movement of the machine element. The unique circuitry designed to control such operation effectively takes advantage of the fact that the velocity may be represented by a rate of change of phase of a command signal.

Under controlled velocity operation, the velocity or feedrate command is represented by a 250 cycle square wave which continually changes in phase at a rate determined by the velocity commanded. This signal is .generated by -means of a feed phase counter 2-16 which is, in effect, :a divide-by-l000 command phase counter similar to the command phase counters 2-11, 2-12, and 2-13; but differing in that it is a variable rate Vcounter which may be controlled to change its counting rate in accordance with desired conditions.

The pulse rate applied to feed phase counter 2-16 via lead 2-40 from the positioning velocity control 2-23 denes the commanded velocity with which the machine tool'is to move. The function of the positioning velocity control circuit 2-23 is to convert a reference pulse rate, applied either directly from a pulse rate divider 2-21 on lead 2-39 or from a manual feedrate override circuit 2-'22 on lead 2-,38, into a pulse rate represented by a number (commensurate with the required velocity of motion) introduced into the system by input data equipment 2-14 via lead 2-37. This control number is therefore inserted from-data equipment 2-14 into the positioning velocity control circuit 2-23 also. In the absence of a velocity command, as when only positioning is important, the output frequency from 2-23 is zero.

An example of the functioning of positioning velocity control circuit 2-23 is available by assuming that a feedrate of 20 inches per minute is commanded. In such a case, the circuit would operate upon the input pulse rate on lead 2-38 or 2-39 to provide an output pulse rate on lead 2-40 of 3.33 kilocycles per second (which is equal to 20 inches per minute with each pulse representing 0.0001 of an inch).

In the event an operator desires to modify this output rate, manual feedrate override circuit 2-22 permits selective modification of the input frequency or pulse rate and thereby affords means for manually performing what, in essence, positioning velocity control circuit 2- 23 does automatically. In passing, it may be noted that pulse rate divider 2-21 may simply be a divide-by-S pulse counter used to accommodate the velocity control section with the other equipment.

Returning to a consideration of the feed phase counter 2-16, it should be appreciated that, in essence, the output of feed phase counter 2-16 is a signal representing a position within the frame of reference of the system. During velocity controlled operation, phase discriminator 2-29 uses the position represented by this signal to command the positioning apparatus. This is accomplished by comparing the feedback signals from the resolvers with this position signal rather than with the command lposition signals. (A convenient name for this signal which is used hereinafter is feedrate-position signal.) By moving the position signal the positioning equipment is continually driven to attain a different goal, i.e. the position indicated, and the rate at which it seeks to attain this goal is determined by the rate at which the goal moves. Accordingly, feed command phase counter 2-16 must accept an input signal that is likely to be high (3 kilocycles per second or higher and develop a 250 cycle per second command signal that continuously changes in phase. It must also provide this phase shift in both positive and negative directions.

The direction of the phase shift is determined by the command larger detector 2-28 that makes continuing determinations of whether the commanded position is a larger or smaller dimension than the present position, in an absolute measuring sense. This comparison is made by means of Hip-flops utilizing first the coarse, then the medium and then the fine command and position signals respectively, for a valid indication of the desired condition. Knowing which is larger, the necessary direction of travel is indicated and thus the direction of phase shift necessary in the feed phase counter 2-16. This information is also used in the phase discriminator 2-29 if the positioning mode of operation is solely involved.

Further means must also be included to utilize the phase shifting feedrate-positioning signal as an input to phase discriminator 2-29 during controlled velocity operation. The feed allowed circuit 2-24 is operative during velocity controlled operation in order to utilize the output of the feed phase counter 2-16 in place of those from the command phase counters 2-11, 2-12, and 2- 13. The end Zone detection circuitry, previously described, operates during the velocity controlled operation also, in order -to discretely define the desired end point. Within 0.080 of an inch of the final position, a comparison is made between the fine command signal and the feed command signal and when synchronism is detected, further positioning is controlled at a rate proportional to the position error as defined by the phase difference between command and position signals. This insures a deceleration to zero at the commanded position from an initial rate as commanded by the velocity con trol signal. Inasmuch as it may take time in the neighborhood of 4 milliseconds to establish the operating conditions following the reading of new information, it is necessary to delay the feed allowed circuit 2-24 until the new command has been established lest a false direction or speed result because of incomplete transition to the new data. Accordingly, the delay circuit 2-25 is included to determine a proper starting time for a feed cycle, following the reading of new tape data.

With the general functioning of the proposed contouring position control system in mind, a more complete understanding will be available from a consideration of specific circuitry designed to perform the desired functions. Of course, equivalent elements may be substituted by those skilled in the art for the particular elements employed. For convenience and brevity of description. the following circuits are individually described in terms of the particular functions they perform. First, however, the circuit symbology used hereinafter will be explained.

DETAILED DESCRIPTION Cizicuz't Symbology Several techniques have been used to make it easier to follow the operation of the illustrative circuit.

For convenience in locating the elements of the circuitry, and as an aid in recognizing the function of these elements, they have been given a two-part designation. In this designation, the numerical prefix represents the figure in which the element appears and the alphabetical suffix is generally an acronym descriptive of the function performed by the particular `circuit element. For example, element 7-SPC is a Hip-Hop in FIGURE 7 which is set to tart the Rhase Qounter. The lead designations and other elements also bear numerical prefixes indicative of the figure in which they originate; however, numerical suixes are used to differentiate between the various elements in each figure.

As a further aid in recognizing the leads over which important control signals are applied, functional lead descriptions are used in addition to the numerical description. These functional descriptions are associated with the appropriate leads by means of small arrows; for example, lead 7-27 in the lower left portion of FIGURE 7 is designated m. This indicates that the command larger signal is applied via this lead. Also, when the bar is placed above this type of functional lead description it indicates that the operative signal is a logic 0. The absence of such a bar indicates that the operative signal is a logic 1.

An additional factor of circuit symbology will be apparent in FIGURE 7. The small circles appearing on particular input leads, of decade 7-10 for example, indicate that the -operative signals applied to these leads must be of the logic 0 state. In the absence of such circles, it is to be understood that the operative signal is of a logic l state.

The convention adopted herein is that a logic value 0 applied on a lead means that a positive voltage is applied. The logic value 1 on the other hand, is represented by a zero or negative voltage. This notation is consistent with the practice followed in the authoritative text on logic switching and design by Keister, Richie, and Washburn, entitled The Design of Switching Circuits, D. Van Nostrand and Co., 1951.

The timing diagrams used in connection with circuit description are illustrated in accordance with the described convention. Thus, the base level, which corresponds to a zero voltage, represents a logic 1; and the raised level, which corresponds to a positive voltage, represents a logic 0. Furthermore, where the state of a flip-flop is represented by a waveform, the signal on the set output is illustrated. When set, the set output has a logic 1 thereat and when reset, the set output has a logic 0 thereat; accordingly, the waveforms shows the set condition as 

1. AN OBJECT POSITIONING SYSTEM RESPONSIVE TO INPUT DATA COMPRISING IN COMBINATION, POSITION COMMAND MEANS OPERATIVE TO PRODUCE TO SAID INPUT DATA TO GENERATE A SIGNAL DISCRETELY REPRESENTING THE ABSOLUTE POSITION SAID OBJECT IS TO ASSUME, POSITION RESPONSIVE MEANS OPERATIVE TO PRODUCE A SIGNAL DISCRETELY REPRESENTING THE ACTUAL POSITION OF SAID OBJECT, VELOCITY COMMAND MEANS OPERATIVE IN RESPONSE TO SAID INPUT DATA TO PRODUCE A SIGNAL DISCRETELY REPRESENTATIVE OF THE VELOCITY WITH WHICH SAID OBJECT SHOULD TRAVERSE THE DISTANCE BETWEEN SAID ABSOLUTE POSITION AND SAID ACTUAL POSITION, FEED MEANS FOR SELECTIVELY MOVING SAID OBJECT AT A RATE COMMENSURATE WITH THE MAGNITUDE OF A SIGNAL, FIRST MEANS CONTROLLED BY THE SIGNALS FROM SAID POSITION COMMAND MEANS AND SAID POSITION RESPONSIVE TO MEANS TO GENERATE A SIGNAL HAVING A MAGNITUDE PORPORTIONAL TO THE DISTANCE BETWEEN THE COMMANDED POSITION AND PRESENT POSITION OF SAID OBJECT, SECOND MEANS CONTROLLED BY THE SIGNALS FROM SAID VELOCITY COMMAND MEANS AND SAID POSITION RESPONSIVE MEANS AND OPERATIVE TO PRODUCE A SIGNAL HAVING A MAGNITUDE COMMENSURATE WITH THE VELOCITY WITH WHICH SAID OBJECT IS TO TRAVERSE, AND SWITCHING MEANS FOR SELECTIVELY APPLYING THE SIGNALS FROM SAID FIRST MEANS AND SAID SECOND MEANS TO SAID FEED MEANS. 